1. Field of the Invention
The present invention relates to an acoustic apparatus in which a vibrator is disposed in a Helmholtz resonator having an open duct port and a cavity.
2. Description of the Prior Art
As a conventional acoustic apparatus in which a vibrator is disposed in a Helmholtz resonator having an open duct port and a cavity, a phase-inversion (bass-reflex) speaker system is known. FIGS. 5A and 5B are respectively a perspective view and a sectional view showing an arrangement of a bass-reflex speaker system. In the speaker system shown in FIGS. 5A and 5B, a hole is formed in the front surface of a cabinet 1, and a vibrator (dynamic speaker unit) 4 constituted by a diaphragm 2 and a transducer 3 is mounted in this hole. An open duct port 6 having a sound path 5 open to an external portion is arranged below the vibrator 4. In the bass-reflex speaker system according to the conventional basic design, a resonance frequency f.sub.op defined by an air spring in the cabinet 1 and an air mass in the sound path 5 is set to be lower than a lowest resonance frequency f.sub.oc of the vibrator when it is assembled in a bass-reflex type cabinet. At a frequency higher than the resonance frequency f.sub.op defined by the air spring and the air mass, the phase of a sound pressure from the rear surface of the diaphragm 2 is inverted at the sound path 5, and hence, a sound directly radiated from the front surface of the diaphragm 2 is in-phase with a sound from the sound path 5, thus increasing the sound pressure. As a result, according to an optimally designed bass-reflex speaker system, the frequency characteristic of the output sound pressure can be expanded below the resonance frequency f.sub.oc of the vibrator. As indicated by an alternate long and two short dashed curve in FIG. 6, a uniform reproduction range can be extended as compared to that of an infinite plane baffle or a closed baffle.
FIG. 7 shows an arrangement of an acoustic apparatus (speaker system with a resonance port) shown the U.S. patent application No. 07/286,869 which was assigned to the same assignee as the present application. In the system shown in FIG. 7, the resonance frequency f.sub.op of a Helmholtz resonator is set to be lower than that of the bass-reflex speaker system shown in FIGS. 5A and 5B. A vibrator for driving the Helmholtz resonator is driven to cancel an air counteraction from the resonator when the resonator is driven. As a result, the acoustic apparatus can be made compact and can reproduce a lower bass sound.
However, in the speaker system with the resonance port according to the application U.S.S. No. 286,869, a distortion of a resonant acoustic wave output from the open duct port cannot always be ignored. The distortion of the resonant acoustic wave occurs more or less regardless of the type of apparatus in every acoustic apparatus in which a vibrator is disposed in a Helmholtz resonator having an open duct port and a cavity like the speaker system with the resonance port according to the application U.S.S. No. 286,869.